1. Field of the Invention
The field of this invention is in the area of computer input devices, including video game controllers.
2. Related Art
As the market for and variety of video games has increased, so has the demand for video game controllers. Early controllers were typically joysticks or trackballs in a fixed mount thus requiring the user or game player to remain fairly stationary while playing the video game. Seeking to give the game player more freedom of movement as well as a greater range of video game input control, still later controllers were handheld and incorporated accelerometers to determine controller movement. Other, external detectors have also been used to detect controller movement. Such external detectors include ultrasound and optical mechanisms that attempt to track movement of a controller in a field of view.
A more advanced form of controller known in the art utilizes magnetic fields. Coils located in a sensor detect the presence of the magnetic field to determine position and orientation of the sensor. Although very precise, such magnetic field trackers typically require close proximity with the transmitter/source of the magnetic field.
One form of such trackers, Alternating Current (AC) electromagnetic trackers, offer advantages for working in a limited physical space or operational volume and are useful for a variety of applications including military (line-of-sight tracking), medical, motion capture, gaming/entertainment, etc. They have no drift in comparison with inertial trackers, are more accurate and sensitive in comparison with direct current (DC) magnetic trackers, and operate beyond line of sight as compared to optical trackers.
However, as is known in the art, one problem of AC electromagnetic trackers is their sensitivity to electromagnetic distortion caused by eddy currents induced by the magnetic field of the transmitter/source in conducting surfaces existing within the operating volume.
This problem can be better understood by first reviewing the well known in the art basic concept of the AC electromagnetic tracker, which is to find the induction vector, Bk, generated by the source/transmitter coil κ(usually there are 3 orthogonal source/transmitter coils, x, y, and z, as is known in the art) at the location of a tracker receiver or probe sensor, with radius vector r from the center of the source to the sensor:
                              B          k                =                                                            IS                source                            ⁢                              μ                0                                                    4              ⁢              π                                ⁢                      1                          r              3                                ⁢                      (                                          3                ⁢                                  (                                                            e                      k                                        ·                    ρ                                    )                                ⁢                ρ                            -                              e                k                                      )                                              (        1        )            where I is the source/transmitter coil's current, Ssource is an effective area of the source/transmitter coilr=|r|, ρ is the unit vector in the direction of r, ek is the unit vector along the axis of kth coil, and μ0 is the magnetic permeability. Then derived is the transfer function, , between the source/transmitter coil's currents and the sensor's voltages that has explicit dependence on their relative position and orientation. Assuming that there are three orthogonal coils per source/transmitter with the same effective area, Ssource, and three orthogonal sensor coils with the same effective area, Ssensor, the electric signal (voltage) in the tracker sensor can be described as:
                              V          =                                                                      S                  sensor                                ⁢                                  S                  source                                ⁢                                  μ                  0                                                            4                ⁢                π                                      ⁢                          ω                              r                3                                      ⁢                                        ·                                                    ,                                  ⁢                              where            ⁢                                                  ⁢                                =                                                    𝕋                ⁡                                  (                                      ψ                    ,                    θ                    ,                    φ                                    )                                                            source                →                sensor                                      ·            ℝ                                              (        2        )            Here V is a 3×3 signal matrix scaled to tracker pre-amp and analog-to-digital-converter (ADC) gains, which is a 3×3 diagonal (ideally) matrix,  is the normalized transfer function that describes the relative position and orientation, actually, the product of the directional cosine matrix,  (ψ,θ,φ)source→sensor, with the position matrix, , that is a 3×3 matrix, each column of which is a vector from the right hand side of the Eq. 1 (in the parenthesis) corresponding to each coil of the source/transmitter, and ℑ is the source's currents matrix, which ideally is diagonal.
As should now be clear to one of skill in the art, the formulas above (Eq. 1 and 2) explain the challenges of maintaining/increasing the operating range of AC electromagnetic trackers while avoiding electromagnetic distortion effects:                Increase of frequency, ω, increases the distortion as ω2         Increase of current, I, or effective areas, S, increases the distortion linearly.        
As is also understood, typical configurations of AC magnetic trackers are known in the art as described in U.S. Pat. Nos. 4,737,794, 4,287,809, 4,314,251, and references therein. However, these trackers have no effective mechanism for the distortion compensation and their performance is limited by the factors described above.
Another known prior approach is described in U.S. Pat. No. 6,539,327 which offers the use of a gradient field for reliable tracking but due to exactly this factor has significantly limited spatial performance in comparison with “conventional” AC trackers.
Several authors proposed combinations of electromagnetic and optical trackers, electromagnetic and inertial trackers, or all of the above. See, for example, U.S. Pat. Nos. 5,831,260 and 6,148,280 and references therein. However, these trackers become more complex from the point of view of hardware and firmware and, while providing additional reference, still have the limitations described earlier. For example, combination electromagnetic/optical trackers will not compensate for the distortion or extend an operational range if optical line of sight is obstructed.
Some patents discuss AC electromagnetic trackers with multiple sources/transmitters. U.S. Pat. No. 6,147,480 discusses a tracker with several transmitters but signal processing requires prior knowledge of the operating environment without the distortion, then, when the source of distortion is introduced, results are compared with a baseline. U.S. Pat. No. 5,752,513 describes a tracker with multiple co-planar transmitter coils where the sensor operates just above the transmitter plane thus confining the operational volume. US Patent publication US2008/0120061 A1 describes a system with multiple wireless miniature transmitters and a stationary sensor. In that system the transmitters are used as probes. Yet this system has no provision for real time distortion compensation and has limited range.
Another system, available from Polhemus and named G4(™), has an ability to work with multiple transmitters/sources; the transmitters are treated one at a time, so the system has no ability to utilize multiple source data for distortion compensation.
Another system is described in U.S. Pat. No. 6,369,564 which has stationary transmitter and sensors and uses a wireless tuned LC circuit as a probe that has a known distortion pattern at a given signal phase. The system has an operational volume restricted by the position of the stationary sensors. One more system is described in U.S. Pat. No. 6,400,139 which has a transmitter, a probe sensor, and an array of stationary “witness” sensors at known fixed positions. Errors in the position and orientation solution for “witness” sensors are used to estimate distortion effects in the probe sensor. As before, this system has the operational volume restricted by the position of the stationary sensors. Further, a system is described in U.S. Pat. No. 6,624,626 which has a single transmitter that emits a modulated, e.g., frequency modulated (FM), signal, and components of this signal are used to compensate for the distortion hut the system has an operational range defined by the transmitter effective area and its drive current.
What is needed is an AC electromagnetic tracker with extended operational range and electromagnetic distortion compensation capabilities without the complexity and additional componentry or complexity of the known approaches.